Starch and starch-derived dextrins commonly have been used for many years as components in adhesives. Typical applications of starch and dextrin adhesives include corrugated box board, envelopes, bottle labeling, paper bag seams, stamps, wallpaper, and laminates. Starch and dextrins have many advantageous attributes for use as adhesives, such as their typical low cost, the abundance of starch, insolubility of starch and dextrins in fats and oils, and lack of toxicity.
In adhesive applications, starches and dextrins provide a good adhesive base, but by themselves are not often used as adhesives in certain applications. For many industrial uses, starches and dextrins are combined with other additives such as polyvinyl acetate, or polyvinyl alcohol. These additives are not only often times derived from petroleum sources, but can significantly raise the cost associated with producing the adhesive.
In particular, in lamination adhesives for multi-wall bags, it is desirable to have an adhesive that adheres well to polyethylene and to paper. There is a large market for multi-wall bags. Many such bags are shipped as a multi-wall laminar structure, the structure including paper and polyethylene, for instance, and sometimes including multiple layers. The individual layers within the multi-wall bag typically are adhered together with adhesive. In such applications, it is desirable for the adhesive to be sufficiently strong to achieve a cohesive bag that will be able to retain the intended contents and that will not delaminate upon filling, handling, and transport. In many embodiments, it is desirable that the adhesive have a “fiber tear” property, such that the adhesive is sufficiently strong to tear a paper or polyethylene substrate upon application of a tearing force.
In light of the wide levels of availability of starch products, starch-based lamination adhesives have been attempted. As starch is modified (typically with acid) to reduce the molecular weight, the solids level of a liquid starch adhesive can be increased. Starch based adhesives have been made in the 30% solids range using this method. Variations incorporate other modifications, including the use of propylated, ethylated, and oxidized starches. These adhesives work well for paper to paper applications, and they may also work in some limited paper to non-porous applications. However, they do not exhibit affinity to polyethylene or to other surfaces that are difficult to adhere. In addition, it is difficult to achieve a workable adhesive with a solids content greater than about 30%. Generally, higher solids contents are more desirable in adhesives, because the liquid component of the adhesive can warp or otherwise damage certain substrates, and a higher solids content generally correlates to a shorter drying time.
Starch derivatives have been used as adhesives. For instance, maltodextrin and syrup solids have been employed. Maltodextrin has an affinity for polyethylene. Additionally, maltodextrin has a low molecular weight relative to starch, thereby allowing for a relatively high solids content (around 65%) in an adhesive. Upon drying, however, maltodextrin becomes brittle. Additionally, maltodextrin has low cohesive strength relative to starch. Hydroxypropyl starch (such as P30L, a starch presently available from Grain Processing Corporation of Muscatine, Iowa) is an adhesive that adheres well to polypropylene, but, for reasons not well understood, does not adhere well to polyethylene.